Add query util read_replica_or_default()

This is similar to use_read_replica_if_available(),
but it just returns either 'read_replica' or
'default', for use with the Django ORM using()
method. Syntactically, it fits nicer with queryset
method-call chains.

common/djangoapps/util/query.py

@@ -4,9 +4,46 @@ from __future__ import absolute_import
 from django.conf import settings
 
 
+_READ_REPLICA_DB_ALIAS = "read_replica"
+
+
 def use_read_replica_if_available(queryset):
     """
     If there is a database called 'read_replica',
     use that database for the queryset / manager.
+
+    Example usage:
+        queryset = use_read_replica_if_available(SomeModel.objects.filter(...))
+
+    Arguments:
+        queryset (QuerySet)
+
+    Returns: QuerySet
     """
-    return queryset.using("read_replica") if "read_replica" in settings.DATABASES else queryset
+    return (
+        queryset.using(_READ_REPLICA_DB_ALIAS)
+        if _READ_REPLICA_DB_ALIAS in settings.DATABASES
+        else queryset
+    )
+
+
+def read_replica_or_default():
+    """
+    If there is a database called "read_replica",
+    return "read_replica", otherwise return "default".
+
+    This function is similiar to `use_read_replica_if_available`,
+    but is be more syntactically convenient for method call chaining.
+    Also, it always falls back to "default",
+    no matter what the queryset was using before.
+
+    Example usage:
+        queryset = SomeModel.objects.filter(...).using(read_replica_or_default())
+
+    Returns: str
+    """
+    return (
+        _READ_REPLICA_DB_ALIAS
+        if _READ_REPLICA_DB_ALIAS in settings.DATABASES
+        else "default"
+    )